The aim of this double-blind, double-dummy, cross-over, randomized, pilot study was to compare the acute bronchodilator efficacy of a single dose of formoterol with that of tiotropium in patients with stable chronic obstructive pulmonary disease (COPD). Because the potential of tiotropium for additive effects is yet unknown, the acute effects of adding this anticholinergic agent to formoterol were also explored. A total of 20 outpatients with stable COPD were enrolled. Single doses of 12 microg formoterol, 18 microg tiotropium, and 12 microg formoterol+18 microg tiotropium were given. Serial measurements of FEV1 were performed over 24 h. Formoterol, either alone or in combination with tiotropium, elicited a significantly faster onset of action and showed a trend for a greater maximum bronchodilation than tiotropium alone. At 24 h, mean FEV1 continued to be significantly higher than pre-dosing value following tiotropium and formoterol+tiotropium. These findings indicate that formoterol and tiotropium have different profiles that make both agents attractive alternatives in the treatment of stable COPD. Since tiotropium ensures prolonged bronchodilation, whereas formoterol adds fast onset and a greater peak effect, the two drugs appear complementary.